1. Field of the Invention
The present invention relates to automatic vehicle location systems, and in particular to dead reckoning vehicle location systems.
2. Description of the Prior Art
Automtic vehicle location (AVL) systems using dead reckoning for determination of vehicle locations are well known. In dead reckoning AVL systems, an initial position of the vehicle is determined and transmitted to a central station. Such initial position or "origin" determination is typically made manually or automatically by a signalling device located at a predetermined location which causes a code to be transmitted to the central station when the vehicle passes in the vicinity of the signalling device. The instantaneous location of the vehicle with respect to the initial position, or origin, is thereafter computed by utilizing directional changes and the distance traveled by the vehicle occurring between the time the vehicle passed the initial position and the time of measurement.
A problem which plagues dead reckoning systems is that distance and heading errors are typically cumulative. Thus, in order for a dead reckoning system to be accurate, the location information must be frequently corrected. Prior art systems have attempted to solve such a problem by maintaining in storage accurate location coordinates corresponding to known drivable surfaces within a predetermined coverage area and approximately correcting the vehicle's location in accordance with a comparison of dead reckoned locations to known locations of drivable surfaces. One such prior art system is described in U.S. Pat. No. 3,789,198, to Henson et al. issued Jan. 29, 1974. In the Henson et al. system each cooperating vehicle is repetitively interrogated at preset intervals, on the order of 5 milliseconds long, and the vehicle responds by communicating to a central station its heading, and the distance traversed since last interrogated. The calculation of dead reckoned position coordinates of the vehicle and the comparison of the dead reckoned coordinates to known coordinates are performed in a computer at the central station. Another such prior art system, developed by Marconi Research Laboratories, is described in U.S. Pat. No. 3,947,807 issued Mar. 30, 1976 to R. A. Tyler et al. and in "UK vehicle tracker is built around microprocessor", Electronics, Mar. 6, 1975, page 48. The Marconi AVM system utilizes a data processor and cooperating map storage means self contained within the vehicle for performing the comparison and correcting function. The Marconi system maintains in the map storage means the distances between all intersections of drivable surfaces within the coverage area, and all angles associated with each intersection as turned through from any possible direction. The dead reckoning system utilizes a turn sensor as opposed to a heading sensor, and the dead reckoned vehicle location is reset to the associated prestored value automatically at each turn. Such prior art systems, are disadvantageous in that large amounts of data storage are required and in that the data processor is relatively complex and expensive. The Henson et al. system is additionally disadvantageous in that it requires a very high rate (density) of communication between the vehicle and the central station to prevent the position error from becoming so large that it cannot be corrected properly.
Another description of the Henson et al. system sometimes known as the Boeing "Flair" System may be found in "A Boeing Plot to Find Squad Cars," Electronics, Feb. 21, 1974, page 30. A description of various prior art systems, including the Henson et al. and Marconi systems, mentioned above, may be found in "Decca to enter chancy vehicle location business", by Michael Payne, Electronics Weekly, Nov. 12, 1975, page 5.
Other prior art systems, as noted in the above mentioned U.S. Pat. No. 3,789,198, have attempted to solve the problem of cumulative errors by establishing a plurality of check points within the coverage area which cause the vehicle to communicate to the central station a predetermined location code as it passes the vicinity of the check point. Such systems are disadvantageous for use with a large number of vehicles in that, as a practical matter, the time of transmission of such predetermined location codes are not controllable and priority conflicts may arise between the respective vehicles. Further, a large amount of communication between the vehicles and the central station is required.